The subject matter disclosed herein relates to use of an incremental angular encoder in a three-dimensional (3D) coordinate measurement device such as a laser scanner, laser tracker, or total station. Such devices steer a beam of light to a target, which may be a cooperative target such as a retroreflector or a non-cooperative target such as a diffusely scattering surface of an object. A distance meter in the device measures a distance to the object, and angular encoders measure the angles of rotation of two axles in the device. The measured distance and two angles enable a processor in the device to determine the 3D coordinates of the target.
The present document discusses the case of a laser scanner, but the extension to a laser tracker or total station will be clear to one of ordinary skill in the art. Laser scanners are typically used for scanning closed or open spaces such as interior areas of buildings, industrial installations and tunnels. Laser scanners are used for many purposes, including industrial applications and accident reconstruction applications. A laser scanner can be used to optically scan and measure objects in a volume around the scanner through the acquisition of data points representing objects within the volume. Such data points are obtained by transmitting a beam of light onto the objects and collecting the reflected or scattered light to determine the distance, two-angles (i.e., an azimuth and a zenith angle), and optionally a gray-scale value. This raw scan data is collected, stored and sent to a processor or processors to generate a three-dimensional image representing the scanned area or object. In order to generate the image, at least three values are collected for each data point. These three values may include the distance and two angles, or may be transformed values, such as the x, y, z coordinates.
Angular encoders are used to measure the two angles of rotation about the two axes of rotation. One type of angular encoder includes a disk and one or more read heads. In an embodiment, the disk is affixed to a rotating shaft, and the one or more read heads are affixed to a portion that is stationary with respect to the rotating shaft. In a type of angular encoder known as an incremental encoder, the disk includes a collection of incremental encoder lines closely spaced at equal intervals along a circle centered on the disk. In addition, an incremental encoder also includes a reference mark, also known as an index mark, which may be a line or a more complicated pattern. Unlike an absolute encoder having a pattern that enables every angular position of the disk to be determined, an incremental encoder provides no such information but rather relies on a serial counting of the incremental lines combined with an initialization based on a signal received from the index mark.
Signals indicating the positions of the incremental lines and the index mark are provided by one or more read heads placed in proximity to the disk. The read head sends out a beam of light that in some cases is reflected off the disk to one or more optical detectors that convert the received light into electrical signals. In other cases, the read heads send out a beam of light that passes through the disk to be received by one or more optical detectors on the other side. In most cases, the angular encoder resolves interpolated angles that are much finer than the angular separation between lines.
For some 3D coordinate measurement devices, the index mark is also used to establish an absolute angular position of the rotating shaft relative to the device. In these cases, there is a need to know whether the absolute angle indicated by the index pulse has changed over time, for example, as a result of a mechanical shock.
Accordingly, while existing 3D coordinate measurement devices are suitable for their intended purposes, what is needed is a 3D coordinate measurement device having certain features of embodiments of the present invention.